Vacuum fuse and method of making same



May 5, 1970 s'. R. SMITH, JR 3,510,819

VACUUM FUSE AND METHOD OF MAKING SAME Filed April 24, 196s w `\`\"`\\``\\r`,"\ m 1 l. M"

United States Patent Gtiice U.S. Cl. 337-246 7 Claims ABSTRACT F THE DISCLOSURE A vacuum fuse manufactured by sealing a fusible element within an evacuated envelope while the envelope is subjected to a relatively high temperature. Subsequent to the sealing operation, the fusible Velement is mechamcally stressed by a coil spring. The stressing force of the spring is correlated `with the tensile strength and thermal characteristics of the fusible element so that this force pulls the fusible element apart when the elements tensile strength is reduced, by the heating effect of overcurrent through the fuse, to a predetermined temperature level. The predetermined temperature level at which such rupture and subsequent fusion of the fusible element occurs is substantially lower than the temperature of fusion of the element when it is not stressed.

This invention relates to vacuum fuses and, more particularly, to a method for making such fuses in a manner such that they can be subjected to relatively high temperatures during their manufacturing cycle wit-hout fusing, and still be adapted to provide a relatively slow timecurrent characteristic.

Although vacuum fuse operating characteristics recommend their use in many applications, the cost of manufacturing vacuum fuses by many prior art manufacturing processes is often so great that it prohibits their utilization. On the other hand, manufacturing methods are known by which vacuum fuses can tbe reliably produced at ia more reasonable cost, but with a sacrifice in fuse operating characteristics. In general, these relatively inexpensive methods of producing vacuum fuses rely on a combination of gettering materials and reducing gas atmospheres during controlled-temperature manufacturing cycles to form the desited vacuums. For example, one such manufacturing method is disclosed in U.S. Pat. No. 2,934,392, De Santis et al., issued Apr. 26, 1960, and assigned to the assignee of the present invention. In la pumpless process such as that taught in this patent, the temperature cycles utilized necessarily subject the fusible element of the fuse to relatively high temperatures, which are determined by the characteristics of the gettering material utilized as well as by the temperature required to hermetically seal the evacuated envelope being formed by brazing it. The presence of such high temperature levels during the manufacturing process makes it necessary to provide the fuse being formed with a fusible element which has a temperature of fusion somewhat above the maximum temperature reached during the manufacturing process. Therefore, although relatively inexpensive vacuum fuses can be manufactured with such a pumpless process, the necessarily high temperature of fusion of their fusible elements has heretofore restricted their use to applications that require fast circuit interruption in response to relatively high overcurrents.

A major advantage of my invention is that it overcomes the above mentioned problems inherent in prior art vacuum fuses by providing an inexpensively manufactured vacuum fuse that has a relatively slow time-current characteristic. Thus, vacuum fuses constructed pursuant to 3,510,819 Patented May 5, 1970 the teaching of my invention provide all of the advantages inherent in presently known vacuum fuses while at the same time incorporating operating characteristics which enable them to be utilized to protect circuits against damage due to low, sustained overcurrents.

Briefly stated, in one form of my invention a vacuum fuse having a thermal-responsive fusible element is provided with spring means coupled to the fusible element to place it under tension. The spring means serves to pull the fusible element apart and initiate its fusion when it is subjected to a predetermined temperature that reduces the tensile strength of the fusible element. The tensioning force of the spring is correlated with the thermal and mechanical properties of the fusible element so that the predetermined temperature at which mechanical rupture is effected is substantially less than the temperature required to fuse the element in its unstressed condition. In manufacturing fuses pursuant to the teaching of my invention, spring tension is applied to stress the fusible element only after the element has been sealed Within an evacuated envelope by a heating cycle that may raise the temperature of the envelope to a level approaching the temperature of fusion of the fusible element in its unstressed state.

Further features and advantages of the invention will become apparent from the following description of the preferred embodiment thereof and from the accompanying drawing, the single figure of which is a side elevation, in cross section, of a vacuum fuse embodying a preferred form of the invention.

yReferring now to the drawing, there s shown a vacuum fuse having a tubular insulating housing 1 which may be formed of alumina or other suitable impervious material. Metallic terminals 2 and 2 are sealed to opposite ends of the housing 1 by a process that will be described further below. In addition to its sealed end cap portion 3, terminal 2 includes a cylindrically shaped housing 4 that is fastened to the end cap 3 -by welding it thereto. On the opposite side, or bottom surface of the end cap portion 3, one end of a metal bellows 5 is sealed in vacuum-tight relation to the end cap 3 and the other end of the bellows 5 is sealed in vacuum-tight relation to the stem portion 6 of an electrode 7. I have found that the vacuum-tight seal between the end cap 3 and metal bellows S can be formed by welding these lmembers together around the circumference of an aperture 3a formed through the center of the end cap 3 to provide a passageway for the electrode stem 6, as will be more fully described below. In order to form a vacuum-tight seal between the metallic lbellows 5 and the stem 6 of electrode 7 in the preferred embodiment of my invention, a plate member 8 is first welded to the stem 6 and then the lower end of the bellows 5 is welded to the outer peripheral edge of the plate member 8. It will be seen that with this arrangement the terminal 2, which includes the end cap 3, the bellows 5, the electrode 7 with its stem portion 6, and plate member 8, serves to form a vacuum-tight seal 4between the housing 1 around the aperture 3a in end cap 3.

A second electrode 9, having a stem portion 10, is electrically and mechanically fastened to the center of end cap 2 by being brazed thereto. An electric circuit is completed between the electrodes 7 and 9 by a thin fusible ele-ment 11, which may be a wire formed of a suitable silver-tin alloy, or other suitable alloy. It will be understood that the dimensions of the fusible element 11, as well as its specific composition, will normally be determined by the time-current rating that the fuse is designed to provide. However, as is more fully discussed below, other considerations such as manufacturing temperature parameters, will necessarily influence the selection of a suitable fusible element 11.

Electrodes 7 and 9 may be of any desired configuration; however, in order to maintain the length of the fusible element 11 relatively short, the electrodes 7 and 9 are formed so that they may be spaced quite close to one another. At the same time, the mass of the electrodes 7 and 9 is kept large with respect to the mass of the fusible element 11, so that the electrodes 7 and 9 act as heat sinks for the fusible element 11 and, thus, make it possible to further shorten the length of the fusible element 11 without increasing its mechanical dimensions or tensile strength. Accordingly, when the fusible element 11 is subject to an overcurrent arc it vaporizes completely and almost instantaneously a very high dielectric is inserted into the circuit between the electrodes 7 and 9 by the substitution of the hard vacuum for the fusible element 11 in the fuse circuit. This simultaneous fusing and introduction of a strong dielectric into the circuit causes the fuse to interrupt the overcurrent which was owing in the circuit, at the lirst current zero.

It will be understood that the fusible element 11 may be fastened to electrodes 7 and 9 in any suitable fashion, but in the preferred form of my invention the electrodes 7 and 9, and their associated stem portions 6 and 10` are slitted so that the opposite ends of fusible element 11 may be inserted, respectively, into the slits, as shown in the drawing at 11a and 11b. The slitted ends of electrodes 7 and 9 are then clamped to the ends-of the fusible element 11 by any conventional crimping process to form a strong mechanical and electrical connection therewith. Pursant to the teaching of my invention, the fusible element 11 is mechanically loaded, to place it in tensile stress, by a coiled spring 12 that is compressed between the cylindrical portion 4 of terminal 2 and a retainer cap 13. The cap 13 is provided with a threaded bore 14, which is adapted to be threaded on the end 6a of the stem portion 6 on electrode 7. When the fusible element 11 is in its unfused condition, as shown in the drawing, it retains the spring 12 in a compressed position so that the spring biases the electrodes 7 and 10' to move away fro-m one another.

The characteristics of spring 12 are correlated with the tensile strength and thermal properties of the fusible element 11, so that the biasing force exerted by the spring 12 on the fusible element 11 will cause it to be pulled apart at a temperature substantially below its temperature of fusion, when the fusible element 11 is subjected to a predetermined temperature level that causes it to be softened sufficiently to reduce its tensile strength. According, by thus spring loading the fusible element 11, the time-current characteristic of the fuse is substantially improved; therefore, the fuse can be used to protect circuits against much slower temperature rises than vacuum fuses have heretofore been capable of satisfactorily protecting.

In manufacturing a fuse according to the teaching of my invention, I have found the following method of manufacture affords a reliable, high quality vacuum fuse at a minimum cost of manufacture. The fusible element 11 is iirst clamped between the electrodes 7 and 9, then the plate member Sis welded to the stern 6 of electrode 7, and bellows and end cap 3 are, in turn, welded together as noted above. The cylindrical portion 4 of terminal 2 may be welded to end cap 3 in a separate operation or simultaneously with the welding operation that seals the bellows 5 to the end cap 3, as is done in the preferred practice of my method of manufacture. After plate member 8 is welded to stem 6, a retaining dish 15 is also affixed to the stem 6 in any suitable manner, such as by brazing, and a disc of titanium hydride 16 is placed in the dish 15. The terminal 2 and the electrodes 7 and 9 assembled therewith are then positioned in the tubular housing 1 and the terminal 2 is brazed to the end of stem 10 on electrode 9. Since I prefer to use the pumpless method of evacuating articles which is described in U.S. Pat. No. 2,934,392--De Santis et al., issued Apr. 26, 1960 and assigned to the assignee of the present invention, to form a vacuum in the envelope defined by the housing 1 and terminals 2 and 2', suitable solder washers 17 and 18 are mounted, respectively, between terminals 2 and 2 and the ends of housing 1. The envelope is then heated to a temperature range between approximately 780 degrees centigrade and 1000 degrees centigrade, dependent on the braze alloy used, and maintained in that range for approximately ten minutes while the envelope is flushed with a hydrogen atmosphere so that the pre-existing atmosphere in the envelope is reduced to substantially pure hydrogen and the titanium hydride disc 16 desorbs most of its hydrogen. As the temperature level increases from 780 degrees centigrade toward 1000 degrees centigrade, after about four or tive minutes the solder washers 17 and 18 melt and form a vacuum-tight seal between the terminals 2 and 2 and and the tubular housing 1. When these vacuum-tight seals are formed, the envelope is allowed to cool, and during this cooling process the titanium hydride disc 16 sorbs substantially all of the gases remaining in the envelope to develop a hard vacuum therein. This vacuum-forming step of my manufacturing process is more fully described in the above-reference patent, including alternative methods for practicing this portion of the process. The important point to note with regard to my method of manufacture is that the thermal characteristics of fusible element 11 must be such that the element 11 does not fuse during the manufacturing process when it is subjected to the high temperatures required by the out-gasing and envelope sealing steps of the vacuum developing process.

Following the formation of the evacuated envelope, the next step in my method of manufacture after the envelope has cooled to a suitable temperature, is to mount the coil spring 12 in the cylinder 4 and compress it between the cup-shaped end cap 13, which is then rotatably threaded on the threaded end 6a at the end of stem portion 6 of the electrode 7. Thus, the fusible element 11 is mechanically loaded under a predetermined mechanical stress by the compression force of coil spring 12. It will be apparent to those skilled in the art that the degree of tension force applied to the fusible element 11 can be linely adjusted by varying the degree of compression of the spring 12 which is determined by rotating it on its thread mounting 6a closer to, or further from, the end cap 3.

In the operation of my invention, it will be understood that the metal bellows and the coil spring 12 need only supply enough reciprocal motion to the electrode 7 to draw it far enough away from the electrode 9 to rupture the fusible element 11 when it is softened by being raised to a predetermined temperature level somewhat below its normal temperature of fusion. However, since the amount of current ilowing through the fusible element 11 to cause it to reach this predetermined temperature level, may be relatively small, the bellows 5 and spring 12 in the preferred form of my invention are adapted to move the electrodes 7 and 9 away from each. other far enough so that the dielectric strength of the vacuum in the envelope is sufficient to interrupt the circuit (following a zero voltage cycle) even if the ends of fusible element 11 are not completely burned back by the relatively low-current arc drawn between them after the mechanical rupture of element 11.

While I have shown and described a specific embodiment of my invention and a method of manufacturing it, I do not desire the invention to be limited to the particular form or method shown and described and I intend by the appended claims to cover all modifications falling within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In an evacuated fuse including a vacuum-tight tubular housing having metallic terminals mounted on opposite ends thereof and a fusible element electrically and mechanically connected between said terminals, the improvement comprising mechanical loading means for placing said fusible element under tension, said fusible element being adapted to sustain the tension exerted by said loading means when subjected to temperatures up to 'a predetermined level and being further adapted to soften and be pulled apart by the tension exerted by said loading means when subjected to temperatures in excess of said predetermined level, wherein one of said terminals cornprises an electrode and a flexible sealing means, said ilexible sealing means being adapted to alford a vacuumtight seal between said tubular housing and said electrode such that the electrode is movable with respect to said housing, the fusible element being connected between the electrode of said one of said terminals and the other terminal, and said loading means being mechanically coupled to said electrode.

2. An evacuated fuse as defined in claim 1 wherein said loading means comprises a coil spring operatively mounted to exert a tensioning force on said fusible element in a direction substantially parallel to the longitudinal axis of said element.

3. An evacuated fuse as dened in claim 1 wherein said mechanical loading means comprises spring means supported in operative relation with respect to said housing, and said flexible sealing means comprises a bellows that affords said electrode an adequate range of movement to prevent an arc being formed between the electrode and said other terminal when the fusible element fuses and said electrode is moved away from said other terminal.

4. An evacuated fuse as dened in claim 3 wherein said electrode is provided with an elongated stem portion, one end of which is coupled to said spring means, said spring means being disposed on one side of said bellows and said fusible element being disposed on the opposite side of said bellows.

5. Fuse means comprising an evacuated envelope, a pair of terminals having relatively movable electrodes thereon, said electrodes being operatively mounted within 'said envelope, spring means mounted on said envelope, mechanical coupling means for coupling said spring means to at least one of said terminals, said spring means being adapted to bias said electrodes away from one another when coupled thereto by said coupling means, vacuum-tight sealing means operatively connecting said spring means to said coupling means without destroying the vacuum in said envelope, a fusible element electrically and mechanically connected to each of said electrodes,

said fusible element being adapted to hold said electrodes in a predetermined spaced apart relationship against the biasing force of said spring means when subjected to temperatures up to a predetermined level and being further adapted to soften and fuse thereby to release said electrodes for movement away from one another responsive to the biasing force of said spring means when said element is subjected to a temperature in excess of said predetermined level, said terminals being adapted to be electrically connected to an electric circuit.

6. Fuse means as defined in claim 5 wherein one of said terminals and its associated electrode is mounted on said envelope in fixed relation thereto, and the electrode on the other terminal is mounted in said envelope in movable relation thereto, means defining an aperture in said envelope, said vacuum-tight sealing means comprising a bellows and means forming a seal between said bellows and, respectively, said movable electrode and the means defining said aperture, said coupling means being adapted to operate through said aperture to mechanically couple said movable electrode through said bellows to said spring;

means.

7. Fuse means as dened in claim 6 wherein said coupling means comprises an integral stem portion of said movable electrode, and said spring means comprises a coil,

spring adapted to be held in a compressed position by said coupling means and said fusible element when said fusible element is unfused.

References Cited UNITED STATES PATENTS 349,190 9/1886 Stanley 337-190 X 1,991,228 2/1935 Rankin 337-246 X 2,292,058 8/1942 Craig 337-246 X 2,518,909 8/1950 Krakauer 337-190 X 3,265,840 8/1966 Luehring 337-246 X HIRAM B. GILSON, Primary Examiner U.S. Cl. X.R. 337- 

